This invention relates to an arrangement designed for biologically activated cleaning of waste waters wherein the separation of activated sludge from cleaned water is accomplished by fluid filtration.
The high efficiency of fluid filtration for the separation of particles of activated sludge having a higher specific weight than water is commonly known. Discrete particles of activated sludge which have a lower specific weight than water and which thus have a tendency for flotation are not returned by the filter mechanism in the fluid layer of the filter, and penetrate into the layer of cleaned water above the level of the fluid filter. Such lower specific weight discrete particles of activated sludge are thus removed together with the cleaned water by the take-off system for the cleaned water. The activated sludge separated by flotation contaminates the clean water, both by an increase in the amount of undissolved materials, and by an increase of the residual organic contamination expressed in values of BSK.sub.5 and CHSK units.
There are a number of different causes of this unwelcome flotation of a part of the activated sludge. One of the main reasons, particularly with apparatus of the column type with a high water column in the activation, is the separation of air bubbles on particles of activated sludge in the separating space. The air bubbles sticking to such particles are capable of reducing their specific weight below the specific weight of water. A similar particle is not retained in the layer of the fluid filter, and is taken along for draining, thus worsening the quality of the cleaned water. The separation of air bubbles is due to the oversaturation of the activating mixture by air in the course of aeration of the activating mixture.
In the case of a very fast vertical circulation of the activating mixture, and in case the separating space is situated in the upper part of the apparatus, all excess air cannot be separated during the course of the upward movement of the mixture wherein a reduction of the hydrostatic pressure takes place. In such a case, the activated mixture enters the separating space with a substantial oversaturated by air. The higher the activating space, the higher is such oversaturation, and thus also the flotation effect due to separated air bubbles is most pronounced with apparatus of the column type. Air from the oversaturated mixture is not separated uniformly. Such separation is particularly marked at places of reduced pressure, for instance, at locations wherein the speed of the liquid flow is increased, where reduced cross-sections are present, or in case of a sudden change of direction flow, for instance around the edge of an inlet opening to the separating space where the direction of flow of the activating mixture changes from a downward direction to an upward direction.
The separation of excess air by said effect is also utilized for degassing the activating mixture, for instance, by the introduction of partition walls in the activating space in front of the separation space; this accelerates the flow or changes in the direction of flow of the activating mixture in the required degree in order to achieve degassing.
The negative influence of the separation of air bubbles at the inlet into the separating space is usually removed by the introduction of a trap for catching floating sludge in the space of the fluid filter; such trap retains the floated sludge and concentrates is in its upper part at the surface. By the use of these means, a substantial reduction of escape of activated sludge into the cleaned water has been achieved. Thus far, the above-mentioned solutions have not been used, the escape of activated sludge, particularly with column arrangements for activation cleaning of water, can achieve such intensity that the amount of activated sludge separated by flotation can be so large as to make impossible the function of the entire arrangement.
Even if the above-described means is employed, it is impossible to prevent separation of air directly in the fluid layer and thus a small amount of activated sludge always has the tendency for flotation, particularly in case of a non-uniform loading of the arrangement, with a sudden hydraulic overload, and thus a certain smaller escape of suspension to the surface of the separating space must always be reckoned with.
Another cause of flotation of activated sludge can be the presence of some material which is lighter than water. The sticking of such lighter material to particles of activated sludge can cause their flotation.
The most intense flotation occurs in case of the non-activation of part of the activated sludge if toxic material starts to be active, or by a change in temperature which causes a change of the population of the biocenology, and the like. In such case, extinct microorganisms, having a tendency for flotation, cause the extraction of a large amount of activated sludge to the surface of the activating space, and its being taken along into the waste, so that a substantial worsening of the quality of the cleaned water is the result.
In order to prevent penetration of activated sludge separated by flotation into the cleaned water, a removal of clean water below the surface is also used. In that case, the sludge separated by flotation accumulates at the surface of the separating space, and forms a floating layer which is not taken along into the off-take of cleaned water sufficiently below the surface.
The cleaned water off-take below the surface has, in addition to an improvement of the cleaning effect, also the advantage of a simplification of the off-take of water, as it is less demanding as to the accuracy of its arrangement than the sensitive system of cleaned water off-take by overflow.
Overflows at the surface are used for the removal of the suspension separated by flotation, by way of which part of the cleaned water including the sludge separated by flotation continuously overflows, and is removed to a pumping pit from which it is returned back into the activating space, whereas the take-off proper of cleaned water is provided below the surface. A drawback of this arrangement is an increased hydraulic load of the separation. If the suspension separated by flotation has to be effectively removed, the overflowing amount of water has to be rather large, and there must be many overflows along the whole surface, so that the suspension separated by flotation can be removed from the whole separating surface. As the maximum intensity of removal of water by way of these overflows is just at the maximum hydraulic load, an increase of the hydraulic peak load due to recirculation of water can achieve one-fifth to one-fourth of the hydraulic load, thus reflecting unfavorably and rather sensitively on the overall capacity of the arrangement.
In addition to the mentioned drawbacks, the system of continuous take-off of sludge separated by flotation has also the drawback that in case of a calamity flotation, for instance in case the condition of activating cleaning passes to another condition with changes of temperature, or if part of the biocenology of the activation is extinct due to toxic material, this take-off of sludge separated by flotation is no longer sufficient, and a removal of the floating sludge layer is in such case rather difficult.